Ken,
If it is with power on only I was thinking there might be some 'leakage' in the wiring that comes from the House. (I figured if it was on the load side of the GFCI out there it would trip) Do you know anything about the wiring method coming from the house, and is it in good condition? Can you disconnect that wiring and try feeding the dock another way temporarily to see if the problem goes away?
Bill
